@each $i in 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 {
  [bga#{$i * 10}] { 
    background-color: rgba(0, 0, 0, $i);
  }
}

@each $style in (primary $background-color-primary, secondary $background-color-secondary, tertiary $background-color-tertiary) {
  [bg-#{nth($style, 1)}] {
    background-color: #{nth($style, 2)};
  }
}